A lot of misguided people like to "upgrade" their car with larger-diameter wheels. There are only two situations when this is legit: if you need larger wheels to fit larger brakes (which is almost never necessary), or if you're trying to hit a sweet spot in terms of tire offerings, which aren't consistent across all sizes. Go for larger wheels for any other reason and you're just slowing your car down.
Now, lighter wheels? That's a fantastic idea. The faster you're going, the more energy that's required to turn your car, and your suspension isn't there to worry about your wheels -- or your brakes, for that matter. Any mass you can save in your wheels will pay huge dividends when you're cornering.